1. Field of the Invention
The present invention relates to a circuit for driving a solenoid, and more particularly to a solenoid driving circuit for driving solenoids used in solenoid valves, solenoid relays and the like at high speed.
2. Description of the Prior Art
For high speed operation of solenoids, there have been proposed various kinds of circuits for supplying a sudden counterelectromotive force to drive the solenoid by cutting off current flowing through a choke coil.
As such a circuit, there is disclosed for example in Japanese Patent Public Disclosure No. 61106/81 an electromagnet driving circuit in which a series circuit of a choke coil and a first switch is connected in parallel with a d.c. power source, one end of the solenoid to be driven is connected with both ends of the choke coil through diodes, and the other end of the solenoid is connected through a switch with one end portion of the d.c. power source.
In this electromagnet driving circuit, the first switch is normally closed so that a steady current is supplied from the d.c. power source through the choke coil, and the switch is normally open. In this condition, when the switch is closed at the same time the first switch is opened, a large counterelectromotive force due to electromagnetic induction is produced in the choke coil. The voltage due to the counterelectromotive force is superposed on the voltage of the d.c. power source to make it possible to provide a sudden impulse of exciting current to the solenoid.
However, the waveform of the exciting voltage supplied to the solenoid by the conventional driving circuit is extremely sharp, has a large peak value, and is extremely narrow in width. Therefore, with respect to other nearby electronic equipment, it constitutes a high level noise source, and the efficiency of the circuit is low since most of the impulse energy is used for the production of noise. The efficiency of the circuit is also low since the exciting energy is provided only for a short time, and the service life of the switches is short since they are subject to voltages having an extremely large peak value. Also, when semiconductor switching elements are used for the switches, it is necessary to use expensive elements which can withstand very high voltages since a voltage with an extremely large peak value is produced in this circuit.
Furthermore, in Japanese Patent Application Disclosure No. 109864/80, there is disclosed a circuit in which a current feedback is provided to the solenoid to be driven for the purpose of improving the response characteristics. However, this circuit has a disadvantage in that a complex circuit structure becomes necessary to provide the current feedback.